docs(handler/serial): add comprehensive English comments to all files

Add package-level architecture documentation describing two modes
(hop-based forwarding and direct proxy), two-tier dialing strategy
in forwardSerial, recorderConn per-packet logging with direction
convention, and metadata parsing details. No code changes.

handler/serial/conn.go

@@ -10,11 +10,28 @@ import (
 	"github.com/go-gost/core/recorder"
 )
 
+// recorderConn wraps a net.Conn to record raw traffic flowing through it.
+// Each successful Read or Write that transfers data is logged via the
+// configured Recorder, optionally annotated with direction markers,
+// timestamps, and hex dumps.
+//
+// The direction convention is:
+//   - Read:  '>' (data entering GOST from the serial port / client side)
+//   - Write: '<' (data leaving GOST toward the serial port / client side)
+//
+// This provides a per-packet-level traffic log, as opposed to the
+// aggregate stats approach used by handlers like redirect/tcp (which
+// accumulate total byte counts and record once at the end).
 type recorderConn struct {
 	net.Conn
 	recorder recorder.RecorderObject
 }
 
+// Read reads from the underlying connection and records the received data.
+// Recording happens only when bytes were actually read (n > 0) AND a
+// Recorder is configured. If Read returns an error alongside data
+// (e.g. io.EOF after the last chunk), the data is still recorded —
+// partial reads before an error are meaningful traffic.
 func (c *recorderConn) Read(b []byte) (n int, err error) {
 	n, err = c.Conn.Read(b)
 
@@ -40,6 +57,10 @@ func (c *recorderConn) Read(b []byte) (n int, err error) {
 	return
 }
 
+// Write writes to the underlying connection and records the sent data.
+// Recording follows the same rules as Read: only when n > 0 and a
+// Recorder is configured. The direction marker is '<' to indicate
+// outbound traffic (data leaving GOST toward the target).
 func (c *recorderConn) Write(b []byte) (n int, err error) {
 	n, err = c.Conn.Write(b)